In the exploration of oil in subterranean formations, logs may be made, via a wellbore in the formation, of the resistivity of the formation typically measured at depths of from about 15 to about 150 inches away from wellbore. Resistivity may be measured while drilling (MWD) the wellbore to geosteer, correlate formations, measure pre-invasion resistivity, pick casing points, and evaluate pore pressures.
In addition to MWD resistivity, the resistivity of formations may also be measured after drilling (MAD), for example, twelve to twenty-four hours after drilling. The time after drilling allows drilling mud in the wellbore to enter into the formation. It can be appreciated that the depth of entry of the mud into the formation will be proportional to the permeability and porosity of the formation, and that the resistivity of the formation will change to the extent that mud has entered the formation. By graphically overlaying the MWD and MAD resistivity logs, a qualitative correlation may be made between the regions of difference, or separation, between the MWD and MAD logs, and the zones of permeability in the formation. The potential productivity zones and the flow profile of a wellbore may then be developed based on the indicated zones of permeability of the wellbore.
While resistivity logs are useful, particularly when the logs are taken from horizontal wellbores, for providing a qualitative indication of the productivity of a wellbore, they do not provide useful quantitative indications of the productivity of a wellbore. It is thus difficult to reliably interpret MWD and MAD resistivity logs to determine the flow profile of a wellbore. Such difficulty results, for example, when the resistivity logs are erratic or are plotted using a non-linear scale, such as a logarithmic scale.
Therefore, what is needed is a method for reliably interpreting MWD and MAD resistivity logs to determine the flow profile of a wellbore.